1. Field of the Invention
The present invention relates to stabilized solid thiocarbonate compositions and methods for producing same.
2. Background of the Invention
The chemistry of thiocarbonic acids and salts has been studied in some detail, as indicated by O'Donoghue and Kahan, Journal of the Chemical Society, Vol 89 (II), pages 1812-1818 (1906); Yeoman, Journal of the Chemical Society, Vol 119, pages 38-54 (1921); Mills and Robinson, Journal of the Chemical Society Vol. 128(II), pages 2236-2332 (1928) and by Stone et al in U.S. Pat. No. 2,893,835, dated Jul. 7, 1959.
According to O'Donoghue and Kahan, as far back as 1826 derivatives of thiocarbonic acid were prepared by Berzelius, who reacted aqueous solutions of hydrosulfides with carbon disulfide, the reactions occurring as in (1) EQU 2KHS+CS.sub.2 .fwdarw.K.sub.2 CS.sub.3 +H.sub.2 S (1)
giving unstable solutions which yielded unstable crystalline salts.
Other thiocarbonates were prepared and further characterized by O'Donoghue and Kahan. Their paper, at page 1818, reports the formation of ammonium thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic acid prepared by dropping a solution of "calcium thiocarbonate" into concentrated hydrochloric acid to produce free thiocarbonic acid (H.sub.2 CS.sub.3). The "calcium thiocarbonate" utilized by the authors is described as a double salt, including the calcium cation in combination with both the hydroxide and the trithiocarbonate anions. In addition to making free thiocarbonic acid, other compounds prepared by O'Donoghue and Kahan included the sodium, potassium, zinc and lead salts. However, regardless of which of these salts were prepared, a common characteristic was their relative instability, with the prepared compounds breaking down and releasing carbon disulfide and hydrogen sulfide and/or a metal sulfide, often in a matter of minutes.
The noted paper by Yeoman reports a further study of thiocarbonates (called trithiocarbonates therein) and also reports the preparation and properties of perthiocarbonates (or tetrathiocarbonates) and derivatives of tetrathiocarbonic acid (H.sub.2 CS.sub.4). Yeoman reports on methods of preparing the ammonium, alkali metal and alkaline earth metal salts of these acid species. For example, Yeoman prepared ammonium trithiocarbonate by saturating an alcoholic ammonia solution with hydrogen sulfide and then adding carbon disulfide to precipitate the product salt. Ammonium perthiocarbonate was prepared in a similar manner, except that after reacting the ammonia and hydrogen sulfide, elemental sulfur was added to form the disulfide, (NH.sub.4).sub.2 S.sub.2 ; adding carbon disulfide immediately precipitated the product.
Yeoman states that "solutions of both ammonium trithiocarbonate and perthiocarbonate are very unstable" due to both decomposition to form thiocyanate as a product, and to "complete dissociation back into ammonia, hydrogen sulfide and carbon disulfide."
Considerable explanation is provided concerning the stability of thiocarbonates, as exemplified by sodium trithiocarbonate and perthiocarbonate. Sodium trithiocarbonate solutions in water are said to remain stable only if oxygen and carbon dioxide are "rigidly excluded"; the presence of oxygen causes decomposition to form carbon disulfide and thiosulfates, while carbon dioxide decomposes the solution to form a carbonate, elemental sulfur, carbon disulfide and hydrogen sulfide. Potassium trithiocarbonate behaves similarly, according to Yeoman.
Yeoman also attempted to prepare and characterize the stability of thiocarbonate salts of four of the alkaline earth metals. Yeoman was unable to prepare a "pure" calcium tri- or tetrathiocarbonate, but did observe that the double salt of calcium trithiocarbonate which he prepared was more stable (probably because it was less hygroscopic) than the sodium or potassium thiocarbonates. The barium tetrathiocarbonate could not be isolated, although Yeoman believed it existed in solution. Solid barium trithiocarbonate was found to be stable, although it was alleged to behave like sodium trithiocarbonate when dissolved in water. The preparation of aqueous solutions of the tri- and tetrathiocarbonate of magnesium and strontium was alleged, but the magnesium thiocarbonates were not isolated.
The previously noted paper by Mills and Robinson shows the preparation of ammonium thiocarbonate by digesting ammonium pentasulfide (obtained by suspending sulfur in aqueous ammonia, then saturating with hydrogen sulfide) with carbon disulfide. A crystalline residue from the reaction was found to be ammonium perthiocarbonate. The authors prepared a "better" ammonium perthiocarbonate product, however, by extracting the ammonium pentasulfide with carbon disulfide in a Soxhlet apparatus.
Stone et al disclose several methods for preparing solid ammonium, alkali and alkaline earth metal salts of tri- and "tetraperoxythiocarbonates," hereinafter referred to simply as "tetrathiocarbonates." One such method involves the solution of an active metal such as sodium in anhydrous ethanol to form an ethoxide which, in turn, is reacted with hydrogen sulfide and carbon disulfide to form sodium trithiocarbonate. They report, however, that the trithiocarbonates tend to be quite soluble in ethanol, and if it is desired to recover the solid material from the solution, it is necessary to treat the reaction mixture with a "displacing agent" such as ether, in which case the thiocarbonates frequently separate, not as solids, but as difficulty crystallizable oils which appear to be saturated aqueous solutions of the trithiocarbonate salt. Consequently, such a procedure is not considered feasible for use on a commercial scale. Similar problems were reported with tetrathiocarbonate salts, which were prepared by reacting a metal sulfide such as sodium sulfide with sulfur and carbon disulfide, using procedures analogous to those for the trithiocarbonates.
These problems were reportedly solved by carrying out the preparation reaction "in a medium which is composed of a major part of a nonsolvent for the reaction components and which contains only a minor proportion, less than sufficient to dissolve the inorganic sulfide, of a liquid which is miscible with said nonsolvent and which is a solvent, to a measurable degree, for the inorganic sulfides." For the reaction medium, the preferred nonsolvents comprise between about 70 and about 90 percent of one or more relatively low boiling hydrocarbon materials such as hexane, cyclohexane and benzene, with the second solvent preferably being between about 10 and about 30 percent ethanol, isopropanol or dioxane. Stone et al report that it is not necessary for the second solvent to be anhydrous and that the "usual" 95-5 commercial azeotrope of ethanol and water is quite satisfactory to produce hydrated salts such as Na.sub.2 CS.sub.3.3H.sub.2 O, although the alcohol produces the aforementioned "oil" when used alone.
Basic physical and chemical properties of these materials and a number of basic method for making them are summarized in considerable detail, starting at page 154 of "Carbon Sulfides and their Inorganic and Complex Chemistry" by G. Gattow and W. Behrendt, Volume 2 of "Topics in Sulfur Chemistry" A. Senning, Editor, George Thieme Publishers, Stuttgart, 1977. However, regardless of which material is made and how it is produced, one common characteristic of the solid salts of tri- and tetrathiocarbonic acid is their relatively poor long term stability and many tri- and tetrathiocarbonate salts will decompose and release carbon disulfide upon exposure to water or air, often within a few hours or even minutes. This is not necessarily bad, if one wishes to use the released CS.sub.2 as a soil fumigant and nematicide. However, where it is desired to provide solid materials for such uses as lubrication or rubber additives, or "dry land" farming, etc., it is necessary that they be produced in one or more forms which provide for and maintain the long term stability of these salts when so used. As disclosed in copending U.S. patent application Ser. Nos. 253,139 and 260,912, one method of stabilizing these salts is to coat the solid particles with an oil or grease. Another is to prepare the salts under completely anhydrous conditions and then store the resultant materials under a dry, inert gas such as argon, hydrogen or, preferably, nitrogen until they are put into use. What is needed are improved methods to prevent their decomposition under ambient conditions. The present invention provides such methods.